Mechanism of Phase Transition in dl-Methionine: Determining Cooperative and Molecule-by-Molecule Transformations

The solid-state phase transition in dl-methionine has been extensively studied because of its atypical behavior. The transition occurs through changes in the molecular conformation and 3D packing of the molecules. Phase transitions in racemic aliphatic amino acid crystals are known to show different behaviors depending on whether conformational changes or packing changes are involved, where the former is thought to proceed through a nucleation-and-growth mechanism in a standard molecule-by-molecule picture, and the latter through a cooperative mechanism. The phase transition of dl-methionine resembles the thermodynamic, kinetic, and structural features of both categories: a conformational change and relative shifts between layers in two directions. The present paper presents molecular dynamics simulations of the phase transition to examine the underlying mechanism from two perspectives: (i) analysis of the scaling behavior of the free energy barriers involved in the phase transition and (ii) a structural inspection of the phase transition. Both methods can help to distinguish between a concerted phase change and a molecule-by-molecule or zip-like mechanism. The free energy predominantly scales with the system size, which suggests a cooperative mechanism. The structural changes draw, however, a slightly more complex picture. The conformational changes appear to occur in a molecule-by-molecule fashion, where the rotational movement is triggered by movement in the same layer. Conformational changes occur on a time scale nearly twice as long as the shifts between layers. Shifts in one direction appear to be less concerted than shifts in the perpendicular direction. We relate this to the edge-free energy involved in these shifts. We believe that the behavior observed in dl-methionine is likely applicable to phase transitions in other layered systems that interact through aliphatic chains as well.

Dehydration of Niclosamide Monohydrate Polymorphs: Different Mechanistic Pathways to the Same Product

Many active pharmaceutical ingredients (APIs) can crystallize as hydrates or anhydrates, the relative stability of which depends on their internal structures as well as the external environment. Hydrates may dehydrate unexpectedly or intentionally, though the molecular-level mechanisms by which such transformations occur are difficult to predict a priori. Niclosamide is an anthelmintic drug on the World Health Organization's "List of Essential Medicines" that crystallizes in two monohydrate forms: HA and HB. Through complementary time-resolved synchrotron powder X-ray diffraction and thermogravimetric kinetic studies, we demonstrate that the two monohydrates dehydrate via distinctly different solid state pathways yet yield the same final anhydrate phase. Water loss from HA via diffusion yields an isomorphous desolvate intermediate which can rearrange to at least two different polymorphs, only one of which exhibits long-term stability. In contrast, dehydration of HB proceeds via a surface nucleation process where simultaneous water loss and product formation occur with no detectable crystalline intermediates. Comparative analysis of the two systems serves to highlight the complex relationship between lattice structure and solid state dehydration processes.

Solid-State Overview of R-Baclofen: Relative Stability of Forms A, B and C and Characterization of a New Heterosolvate

A new polymorphic form (Form C) of enantiopure Baclofen was isolated and characterized. Crystal structures of R-Baclofen Form A and Form C were resolved from powder diffraction data, and cell parameters by profile matching for Form B. The relative stability of these three forms is proposed based on structural data, thermal analyses and solvent-mediated conversions. The experiments highlight the stability order A < C < B at 25 °C (A is the most stable form), whereas above 180 °C it would likely be: C < A < B (C being the stable modification). Moreover, a new heterosolvate of the molecule is observed in N,N-DMF/water mixture. This heterosolvate offers a new pathway to isolate pure R-Baclofen Form B provided the lactam impurity does not exceed 3%. Upon mechanical stress Form B tends to evolve to Form C.

Influence of intermolecular interactions and crystal structure on desolvation mechanisms of solvates

The desolvation processes of the solvates were systematically investigated by various analytical techniques and were analyzed based on the intermolecular interactions, molecular networks, void types, proportion of voids and packing patterns.

R. S, Gonnade RG. Polymorphs and hydrates of the anticancer drug erlotinib: X-ray crystallography, phase transition and biopharmaceutical studies

The anticancer drug erlotinib revealed two polymorphs and two hydrates. The metastable polymorph and hydrates converted to the stable polymorph, which displayed solution-mediated transformation into the monohydrate at the lowest water activity.

Organic solvates in the Cambridge Structural Database

Data informatics methods applied to the Cambridge Structural Database reveal shifting trends in solvate formation and inherent biases in the symmetry and packing fraction of solvates and their solvent-free analogues.